1. Field of the Invention
This invention relates to a twisted nematic (TN) liquid crystal display device having an enhanced multiplex driving characteristic.
2. Prior Art
A schematic sectional diagram of a conventional twisted nematic (TN) liquid crystal display device is shown in FIG. 5. In this diagram, in order to orient so that nematic liquid crystal molecules may be formed in a twisted spiral structure between two electrode forming substrates a, b, for example, the oblique evaporation method or the so-called rubbing method of rubbing the surface of electrode substrates contacting with the liquid crystal in one direction by using a cloth may be employed. When the two electrode substrates a, b thus oriented are opposedly placed so that the orientation directions may be orthogonal to each other as shown in FIG. 6, and nematic liquid crystal g having a positive dielectric antisotrophy is injected into their gap, the liquid crystal molecules are arranged in a spiral structure rotated 90.degree. between the electrode substrates a, b. In FIG. 6, "j" denotes the rubbing direction of upper electrode substrate a, and "k" indicates the rubbing direction of lower electrode substrate b. Polarizing plates h, i are disposed above and beneath the liquid crystal cell composed in this way, in which the direction of axis of polarization l of the polarizing plate h and the direction of axis of polarization m of the polarizing plate i are arranged so as to coincide nearly with the polarizing directions of the upper and lower electrode substrates a, b respectively (see FIG. 6). Meanwhile, in FIG. 5, c, d are transparent electrodes, and e, f are deflecting films.
Incidentally, when such conventional matrix type liquid crystal display device is driven by the optimum voltage averaging method of count number N, the ratio .alpha. of the effective voltage at lit picture element Vrms(ON) to the effective voltage at unlit picture element Vrms(OFF) is expressed as follows: ##EQU1## In this equation, if N.fwdarw..infin., then .alpha..fwdarw.1. That is, as the count number N increases, the difference between Vrms(OFF) and Vrms(ON) decreases. Therefore, to realize a matrix type liquid crystal display device with a huge quantity of display information, in order to obtain a favorable contrast ratio if the voltage difference of Vrms(ON) and Vrms(OFF) is small, it is required to develop a liquid crystal material possessing a sharp threshold characteristic or improve the orientation film. Recently, in particular, in the field of liquid crystal display device, as the display information increases, there is a mounting demand for an expansion of the quantity of display information. However, in such conventional TN liquid crystal display devices, it is not flexible enough to cope with increase of the count number N.
Accordingly, as a method of improving the above point, an attempt to improve the multplex driving characteristic by setting the twisting angle .phi. of the liquid crystal molecules larger than 90.degree. has been proposed. That is, in order to twist the liquid crystal molecules more than 90.degree., an optical active substance to induce the twisted spiral structure of liquid crystal molecules is added more to the nematic liquid crystal, and the content of the optical active substance is adjusted so that the ratio d/p of the resultant spiral pitch p of nematic liquid crystal to the liquid crystal layer thickness d may nearly satisfy the relation of d/p.apprxeq..phi./360.degree..
However, in such a liquid crystal display device having a high content of an optical active substance, the orientation of the liquid crystal molecules was disturbed when a voltage was applied, and the light scattered about as a result to lower the contrast ratio. To solve this problem, it was also attempted to form an orientation film by oblique evaporation and extend the angle formed by liquid crystal molecules and electrode substrates, that is, the pretilt angle, but in this oblique evaporation method, it is not easy to form a uniform orientation film over a wide area and it was difficult to obtain a liquid crystal display device of a wide area. Besides, in this method, since it was difficult to orient uniformly and simultaneously on plural electrode substrates, it was difficult to apply in a mass production system.